(1) Field of the Invention
The present invention relates to a power plant for an aircraft, in particular a rotorcraft, and to a method of piloting said aircraft.
(2) Description of Related Art
Most presently-manufactured rotorcraft have one or two turboshaft engines. Power is then taken from a low pressure turbine referred to as a “free turbine” that is mechanically independent of the engine assembly comprising a compressor and the high pressure stage, and in particular including a high pressure turbine. The free turbine of an engine generally rotates at 20,000 revolutions per minute (rpm) to 50,000 rpm, so a speed-reducing gearbox is needed for the connection with the main rotor of the rotorcraft since its speed of rotation lies substantially in the range 200 rpm to 400 rpm: this is the main power transmission gearbox referred to more simply as the main gearbox (MGB).
Thermal limitations of an engine and torque limitations of a main gearbox serve to define a utilization envelope for the engine covering at least two normal utilization ratings of an engine mounted on a single-engined or twin-engined rotorcraft:
a takeoff rating corresponding to a torque level for the main gearbox and to heating of the engine that are acceptable for a limited length of time without significant degradation, this takeoff rating being defined by a maximum takeoff power PMD and a duration of utilization of the maximum takeoff power that is generally of the order of 5 minutes; and
a maximum continuous rating, the maximum continuous rating being defined by a maximum continuous power PMC corresponding to about 90% of the maximum takeoff power PMD and by a duration of utilization for the maximum continuous power that is generally unlimited.
On a twin-engined rotorcraft, the utilization envelope also covers super-contingency ratings, that are used only when one of the two engines has failed;
a first contingency rating, this first contingency rating being defined by a super-contingency power that is often equal to about 112% to 120% of the maximum takeoff power PMD and by a duration of utilization of this super-contingency power that is generally about thirty consecutive seconds at the most, known as one engine inoperative thirty seconds (OEI30″) rating, which super-contingency rating is conventionally usable three times during a flight;
a second contingency rating, this second contingency rating being defined by a maximum contingency power PMU equal to about 105% to 110% of the maximum take off power PMD and by a duration of utilization of this maximum contingency power PMU of the order of two consecutive minutes at most (OEI2′); and
a third contingency rating, this third contingency rating being defined by an intermediate contingency power that is substantially equal to the maximum takeoff power PMD and by a duration of utilization of this intermediate contingency power that is unlimited for the remainder of the flight after the failure of an engine (OEIcont).
Thus, the engine manufacturer defines a utilization envelope for the engine, this utilization envelope comprising a plurality of ratings, each rating associating a level of power developed by the engine with a duration of utilization for that power. Use is sometimes made more simply of the term “envelope” or of the expression “performance envelope” to designate such a utilization envelope.
The power differences between the various ratings may also be referred to as “power staging” of the engine.
Furthermore, the thermal and mechanical constraints and above all the phenomenon of the turbine blades creeping can lead to the engine being degraded to a greater or lesser extent depending on the rating. In order to guarantee both safety in flight and also that performance is achieved, the maximum acceptable damage for an engine is determined.
Thereafter, the potential overall utilization of the engine is evaluated. Concretely, this amounts to defining a maximum number of hours of flight, referred to as time between overhauls (TBO) by the person skilled in the art, that the engine is capable of performing from its most recent overhaul or from its first utilization, depending on the circumstances applicable. Once this maximum number of flying hours has been reached, the engine is removed from the aircraft and then overhauled.
Thus, the engine manufacturer defines a utilization envelope for the engine that is associated with a maximum number of flying hours, the utilization envelope being made up of a plurality of ratings, each rating associating a power developed by the engine with a duration of utilization of that power. The engine manufacturer also associates a maximum number of flying hours with the utilization envelope.
It should be recalled that a turbine engine is usually associated with control means, and the information relating to the ratings of a utilization envelope is stored in the control means. Under such circumstances, when the pilot of an aircraft requires a given rating to be used, the control means control the engine and in particular its fuel metering pump so that the engine responds to the order given.
Furthermore, in order for a rotorcraft to obtain authorization to fly in any given country, it will be understood that the utilization envelope and the maximum number of flying hours of the engine(s) of the rotorcraft need to be certified by the official services of the country in question for a specified spectrum of utilization. Such authorization is therefore obtained only after full certification testing, e.g. including an endurance test.
Such complete certification testing of an engine is performed in order to justify a utilization envelope associated with a maximum number of flying hours. It is then not permissible to use the engine with a utilization envelope that is different from the initially authorized utilization envelope, without performing additional complete certification testing, which is very expensive.
It can be understood that a given engine may correspond to a particular type of mission. Nevertheless, the engine is in danger of not having optimized staging for the ratings of its utilization envelope if it is to be used with a different type of mission.
For example, a rescue mission with winching requires an engine to operate with a utilization envelope that is different from a utilization envelope that has been optimized for a mere ferrying mission.
The power levels of the contingency ratings are even more sensitive when they are used as a result of an engine failing.
Thus, after an engine failure during takeoff from a spot heliport, it is advantageous to have a high level of OEI30″ super-contingency power available in order to keep the aircraft in flight, rather than a high level of intermediate contingency power OEIcont.
Conversely, during a stage of cruising flight on instruments or a takeoff from open terrain, it is advantageous to have a high level of intermediate contingency power OEIcont.
Under such circumstances, a utilization envelope enables one type of mission to be performed, but does not, a priori, enable some other type of mission to be performed, or at least not in optimized manner.
A manufacturer thus defines the utilization envelope for an engine by making compromises as a function of the missions to be performed.
A power plant is also known that has at least one engine and corresponding means for controlling the engine. The control means comprise a memory containing information to cause the engine to operate with at least two distinct utilization envelopes during a maximum number of flying hours, each utilization envelope having at least two distinct utilization ratings, each defined by a developed power and by a duration of utilization of that developed power, said at least two utilization envelopes comprising a utilization envelope for optimizing takeoff from a platform, and another utilization envelope for takeoff in optimized manner from takeoff zones that do not include platforms.
With the help of selection means, a pilot can select which utilization envelope to apply.
According to document FR 2 878 288, it is possible to modify a utilization envelope of a turbine engine by modifying the maximum number of flying hours.
According to document FR 2 878 288, starting from an initial utilization envelope, an alternative utilization envelope is established. The changeover from the initial utilization envelope to the alternative utilization envelope is performed without modifying the maximum number of flying hours of the engine but by lowering the value of a parameter of the initial utilization envelope. For example, the power of a given rating is increased but the duration of utilization of that rating is shortened.
The state of the art also includes documents EP 1 281 846 and FR 2 602 270 that mention the possibility of re-evaluating the limits of an engine in the event of an emergency.
Also known are the following documents: U.S. 2009/0186320, U.S. Pat. No. 5,873,546, WO 99/51868, EP 0 816 226, and FR 2 902 408.
According to document U.S. 2009/186320 A1, an available power margin is determined and then a variable bias relative to the power margin in order to simulate reduced power. Under such circumstances, it is possible in equivalent manner to reduce the power developed by each engine or to act as a function of the weight conditions of the aircraft.
Thereafter, document U.S. 2009/186320 A1 describes an operating envelope and introducing bias in order to simulate a failure.
Document U.S. Pat. No. 5,873,546 A also suggests introducing bias.
Document WO 99/51868 A1 relates to a method and a device for controlling the thrust of an aircraft with the help of a single lever.
Document EP 0 816 226 A1 describes a cockpit indicator. FIG. 5 shows a curve illustrating flight stages.
Document FR 2 902 408 A1 describes a method of balancing two turbine engines.